For some years now this Institute has used the method of ultrasonic fractography to measure fracture speeds.
In this method a high-intensity ultrasonic shear wave is transmitted through the specimen during fracture, in a direction normal to the expected fracture plane, producing a light sinusoidal ripple on the fracture surface. Up to now it has been applied chiefly to glass.
Recently we have been able to apply the method to single crystals of silicon and germanium. These substances fracture both crystallographically, on the (iIi} planes, and on non-crystallographic surfaces. Abraded and lightly notched specimens have been tested in simple threepoint bending, which produces the least loss of ultrasonic intensity to the grips. The frequencies used correspond to wave lengths of about 1 ram, less than the crystal dimensions. Sinusoidal modulations produced by the ultrasonic wave are easily visible on the non-crystallographic fracture surface. None are observed on the {Iii) planes. Speeds of between 20 and 2000 m/s have been measured and agree with those calculated from the Wallner lines. Dragging of the crack is observed at steps caused by lancet-fracture and at an abraded surface. The method is being used to trace the acceleration and deceleration of the crack during the bending process.